1. Field of the Invention
This invention relates generally to crystal growing systems utilizing a melt within a crucible and a crystal seed that is withdrawn therefrom. More particularly, this invention relates to a method and apparatus for detecting the level of the melt within the crucible.
2. Background Discussion
Several techniques are known in the art for growing crystals. The Czochralski process is the most widely used of these processes and is generally summarized below. A heated crucible is provided for holding a melted form of a charge material from which the crystal is to be grown. The melt is maintained at a temperature slightly above that at which the charge material crystallizes, A seed is placed at the end of a cable or rod that enables the seed to be lowered into the melt material and then raised back out of the melt material. The seed can be either a sample of the desired crystal material, or any other compatible material that has a higher melting temperature and the same crystalline structure and parameters. When the seed is lowered into the melt material, it causes a local decrease in melt temperature, as is known to those skilled in the art, which results in a portion of the melt material crystallizing around and below the seed. Thereafter, the seed is slowly withdrawn from the melt. As the seed is withdrawn, the portion of the newly formed crystal that remains within the melt essentially acts as an extension of the seed and causes melt material to crystallize around and below it. This process continues as the crystal is withdrawn from the melt, resulting in crystal growth as the seed is continually raised.
A primary goal of crystal growing systems is to grow crystals that have uniform properties over their entire length. In order to achieve this goal, it is desirable to ensure that the growing conditions for the crystal remain constant throughout the growth process. A number of factors can influence the growing conditions of the crystal. In a system utilizing the Czochralski process, the crucible is located within a furnace that heats the crucible only around its exterior surface. During crystal growth, the melt level in the crucible changes. These changes of the melt level within the crucible result in changes in the thermogradients at the melt/crystal interface. Any change in the thermogradients during the growth of a crystal is undesirable because it changes the growing conditions for the crystal, thereby resulting in a non-uniform crystal. Two types of systems have been utilized that attempt to maintain uniform growing conditions in different ways. First, one type of system raises the crucible along with the crystal. These systems attempt to match the rate at which the crucible is raised to the rate at which the melt volume decreases so as to maintain the melt level at a constant vertical position relative to the furnace. In this manner, changes in the thermogradients due to decreased melt volume are reduced. Second, another type of system replenishes the melt as the crystal is withdrawn in an attempt to maintain a constant melt volume.
In each of the above-described systems, it is important to accurately determine the location of the melt level within the crucible so that either: (1) the crucible is raised by the appropriate amount; or (2) the amount of any decrease in melt volume can be detected in order to determine the appropriate amount of replenishing material to be added to the melt to restore it to its proper volume. Several prior art systems have been utilized for detecting the melt level. First, some systems have determined the melt level indirectly by determining the rate at which the crystal is withdrawn from the melt. These systems assume that the melt volume will decrease at a rate that is equal to the growth rate of the crystal. Therefore, they utilize the rate at which the seed is withdrawn from the melt order to estimate the growth rate of the crystal. However, this type of system allows only a crude and inexact determination of the melt level because it does not monitor the melt level directly. As previously stated, it is desirable to determine the melt level as precisely as possible in order to provide uniform growing conditions for the crystal.
Second, another type of prior art system utilized to detect the melt level is best described by making reference to FIGS. 1 and 3. A light source 1 is provided outside of the crucible and located to one side thereof. The light source provides a beam of light that is directed toward the melt. The beam of light reflects off of the melt and then upward toward a light detection system 3 that is located outside of the crucible and on the other side thereof. By analyzing the location of the reflected light beam, the light detection system 3 determines the position of the melt level.
For reasons that are more fully described below, the detection of the melt level in this second type of system is complicated by the fact that the melt surface is constantly interrupted by waves traveling across it. These waves are caused by a number of factors, some of which can be effectively minimized, and others that cannot. Some of the wave activity is due to mechanical movements that can be somewhat minimized through proper system design. For example, crystal growing systems generally rotate the crucible in order to ensure uniform heating of the melt. Furthermore, most systems either: (1) raise the crucible as the crystal is withdrawn from the melt material; or (2) add charge material to the crucible in order to replenish the melt as the crystal is withdrawn. Each of the above-described mechanical causes contributes to waves being formed across the surface of the melt.
Although some of the above-described mechanical effects can be minimized, there is another factor that contributes to the disruption of the melt surface that cannot be effectively minimized. In systems used to grow silicon crystals, an important consideration is preventing the introduction of contaminating material into the melt because it can lead to non-uniformities in the crystal, or change its properties thereby rendering it useless. There are a limited number of materials that are non-contaminating with respect to a melt of silicon material. Of these materials, the one that is generally used to line the interior of the crucible is SiO.sub.2. Because this material is a chemical compound of silicon, the materials have an affinity for one another when they come into physical contact. As a result, waves are generated due to constant wetting and dewetting of the crucible. Although these processes are well-known to those skilled in the art, they are briefly summarized below.
Because the silicon melt material has such a strong affinity for the SiO.sub.2 material along the interior of the crucible, it undergoes a constant wetting process where the silicon material attaches itself to the side of the crucible. During this process, the silicon material can even rise upward against the force of gravity and attach to the walls of the crucible. As a result, the melt level is briefly reduced. However, the surface forces responsible for this phenomenon are strongly influenced by the temperature and melt composition. Thin layers of melt are much more susceptible to temperature and composition changes. Such changes can perturb the balance of surface tension forces and cause a shift from wetting to a process of dewetting where the silicon that is attached to the sides of the crucible disengages from the sides of the crucible and reenters the melt. This perpetual wetting and dewetting causes fluctuations in the melt volume and causes waves to be formed across the surface of the melt. This effect cannot be practically prevented. Therefore, even if all the mechanical sources of waves are essentially eliminated, the melt surface will nevertheless be constantly interrupted due to waves formed due to the wetting and dewetting processes.
As previously stated, the fact that the melt surface is constantly interrupted by waves moving across it creates complications for melt detection systems utilizing a beam of light that is reflected off of the melt surface. The angle at which the light beam contacts the melt surface will vary depending upon what, if any, portion of a wave the beam contacts. Because waves are constantly moving across the melt surface, the angle at which the light beam contacts the melt surface will vary over time in a somewhat random manner. As the angle of contact between the light beam and the melt surface varies, the path of the light beam reflected off of the melt surface similarly varies, as shown in FIG. 1. Consequently, over a given time period, the reflected light beam moves about within a given area where its position can be detected as is shown in FIG. 2. The light pattern 5 represents the time elapsed position of the reflected light beam over a period of time and is essentially a random pattern that falls within an area 7. The light detection system must therefore be able to account for the fact that the light beam may be incident anywhere within the area 7 and that the area 7 may be rather large.
Prior art light detection systems have utilized detectors made from a number of discrete sensors as shown in FIG. 3. These prior art systems have utilized the discrete sensors in two basic configurations, i.e. as two-dimensional (2-D) area detectors such as the one shown in FIG. 3a, or as one-dimensional (1-D) array line detectors where a single line of sensors is utilized such as the one shown in FIG. 3b. The use of discrete sensors to form such detectors has a number of disadvantages. First, each sensor can be somewhat expensive. In this regard, the larger the area over which the reflected light beam may be incident, the more sensors that are necessary to cover that area, thereby increasing the cost of the detector system. Some prior art systems have utilized an optical lens positioned between the melt and the detector for the purpose of focusing the reflected light into a smaller area. However, even in such systems, a significant number of discrete sensors are required. Additionally, the use of an optical lens to reduce the detection area does not resolve any of the other disadvantages associated with the use of multiple discrete sensors that are described below.
Second, an array of small sensors is fragile in that each sensor is susceptible to becoming inoperative. In this regard, if any one of the sensors fails to operate properly, the sensor array necessarily functions improperly and cannot accurately detect the melt level. The larger the area over which the reflected light beam may be incident, the larger the sensor array must be and, correspondingly, the larger the number of sensors that must be utilized, thereby increasing the probability of a sensor becoming inoperative.
Third, the use of detectors made from discrete sensors creates some imprecision in the system, the degree of which is dependent upon the size of the individual elements which determine the resolution of the detector. Each sensor that is illuminated by the reflected light beam sends a notification signal to a control circuit. However, there is no way to determine precisely where, on such a sensor, the light beam is incident. Therefore, if large sensors are utilized, the melt level determination is imprecise because the precision with which the melt level can be detected is limited by the size of the sensors. As previously stated, it is desirable to determine the melt level as precisely as possible so as to minimize changes in the growing conditions, thereby enabling the growth of uniform crystals.
Accordingly, it is an object of the present invention to provide an improved method and apparatus for determining the melt level within a crucible utilized in a crystal growing system.